Partly fixes <https://bugs.gnu.org/40574>.
Reported by Jan Nieuwenhuizen <janneke@gnu.org>.
Previously, we'd choose at expansion time whether to use the Hurd or the
Linux variant, taking the cross-compilation target into account. This
would lead to the wrong decision when (guix build syscalls) is evaluated
while we're cross-compiling to GNU/Hurd.
This is a followup to 1ab9e483391f8b62b873833ea71cb0074efa03e7.
* guix/build/syscalls.scm (define-generic-identifier)
(read-dirent-header, %struct-dirent-header, sizeof-dirent-header):
Remove.
(readdir*): Rename to...
(readdir-procedure): ... this, and add parameters.
(readdir*): Define as a call to 'readdir-procedure' as a function of
%HOST-TYPE.
82d8959e5d